The present invention is directed to a numerical control machine tool system and, more particularly, to apparatus for averaging of two position feedback signals representing position of a single axis of motion.
In numerical control machine tool systems a position transducer is generally associated with each axis of motion of the machine. The transducer provides a feedback signal to a numerical control from which signal the numerical control derives information indicating the position and velocity of the machine tool with respect to a work piece. This information is then utilized by the numerical control in generating position and velocity commands to be applied to motors controlling movement of the machine. Such a numerical control system is shown in U.S. Pat. No. 3,173,001--Evans, assigned to the assignee of the present invention. In Evans the feedback signal is provided by X and Y axes position feedback synchro resolvers. The feedback signal from each resolver is applied to a corresponding pulse shaper circuit which converts the sine wave output from the resolver into a square wave while maintaining the proper zero cross overpoint of the sine wave such that phase information is retained. The squared feedback signal from the pulse shaper circuit is then applied to a digital phase discriminator which includes a reversible counter. The phase discriminator compares the phase of the feedback signal to the phase of the commanded position signal and produces a digital output error signal proportional to the phase difference. This digital output error signal is converted to an analog signal which is applied by means of a servo amplifier to drive the machine to the respective axis.
In some machines the construction and operation are such that it becomes impossible to locate a single position transducer in a manner to detect position and motion of the machine tool along a particular axis. Such a situation might occur, for example, in a large milling machine in which the bed of the machine is moved along the vertical axis by means of gears located at opposite ends of the bed. Because of the inability to make perfectly matched gears it is obvious that in this type of milling machine movement of one end of the bed will not correspond exactly with movement of the opposite end of the bed and that movement of the work piece with respect to the machine tool will not correspond exactly with either end of the bed.